Capacitance type material detectors are well known as means for detecting the level of material in a hopper or bin, and their use in hoppers for particulate solid material is well established. An electrode in a support housing is inserted in the hopper wall, and the electrode and the hopper wall create an effective capacitance device. The capacitance value is a function of the presence or absence of material in the hopper, and is measured by a remotely located current sensing circuit which may be conveniently termed a capacitance unit. Such units are commercially available from several sources. The particular detector structure here disclosed has been used with a capacitance unit supplied by Robertshaw Controls.
Prior art capacitance type material level detectors have certain deficiencies which are eliminated by the apparatus of the present invention. We have learned that variations in moisture content or chemical composition of the stored material can produce deceptive readings by varying the capacitance value even though the same amount of material is in the hopper. The present invention eliminates this problem.
In addition, existing systems lack any simple means of checking to determine whether the device is functioning; and the present invention provides such means.
In addition, certain types of particulate material such as very fine powders can accumulate on the face of the detector unit and cause a false indication of the presence of material. The apparatus for verifying the functioning of the device is so arranged that it may also be used to break accumulated material from the face of the detector unit.